Econintersect: There are two varieties of tuna in the eastern Pacific that are caught off the California coast: Bluefin and yellowfin tune. The bluefin variety is 
also found in the western Pacific off the coast of Asia. Bluefin tuna caught off the coast of California in August 2011 have been found to contain elevated levels of radioactive elements associated with the Fukushima nuclear plant meltdown disaster that followed the 11 March 2011 earthquake and tsunami in Japan. Note: The levels of radiation in the tuna are not considered a health risk to humans who would consume them. The levels of radiation from the radioactive cesium connected to the Fukushima reactors were far less than other naturally occurring radioactivity existing in all fish.The importance of the finding is that it shows that tuna can disperse material from the Japanese coast to the California coast in just five months time. The determination that the increased cesium radioactivity came from Japan was made by comparing to the bluefin tuna caught before the disaster, as well as to yellowfin tuna caught before and after. The three control samples do not show elevated cesium radiation. Note: The yellowfin tuna never leave the eastern Pacific so they could never have been near Japan.
The research reults were published in a paper in the Proceedings of the National Acadamy of Sciences of the United States, 29 May 2012. Abstract of the paper:
The Fukushima Dai-ichi release of radionuclides into ocean waters caused significant local and global concern regarding the spread of radioactive material. We report unequivocal evidence that Pacific bluefin tuna, Thunnus orientalis, transported Fukushima-derived radionuclides across the entire North Pacific Ocean. We measured γ-emitting radionuclides in California-caught tunas and found 134Cs (4.0 ± 1.4 Bq kg−1) and elevated 137Cs (6.3 ± 1.5 Bq kg−1) in 15 Pacific bluefin tuna sampled in August 2011. We found no 134Cs and background concentrations (∼1 Bq kg−1) of 137Cs in pre-Fukushima bluefin and post-Fukushima yellowfin tunas, ruling out elevated radiocesium uptake before 2011 or in California waters post-Fukushima. These findings indicate that Pacific bluefin tuna can rapidly transport radionuclides from a point source in Japan to distant ecoregions and demonstrate the importance of migratory animals as transport vectors of radionuclides. Other large, highly migratory marine animals make extensive use of waters around Japan, and these animals may also be transport vectors of Fukushima-derived radionuclides to distant regions of the North and South Pacific Oceans. These results reveal tools to trace migration origin (using the presence of 134Cs) and potentially migration timing (using 134Cs:137Cs ratios) in highly migratory marine species in the Pacific Ocean.
Sources:
- Pacific bluefin tuna transport Fukushima-derived radionuclides from Japan to California (Daniel J. Madigana, Zofia Baumann, and Nicholas S. Fisher, Proceedings of the National Acadamy of Sciences of the United States, 29 May 2012)
- Bluefin Tuna Carry Radiation From Japan’s 2011 Fukushima Nuclear Disaster (RedOrbit Staff & Wire Reports, 29 May 2012)
- Tuna in US found with Japan quake radiation: study (AFP, 29 May 2012)
